This invention relates generally to equalizer circuits and specifically to equalizer circuits that are used in High Definition Television (HDTV) receivers.
In the recently adopted digital advanced television systems standards, there will be a relatively long period when both NTSC and HDTV signals may be broadcast in the same service areas. The HDTV receivers will incorporate a filter, such as a comb filter, which cooperates with precoding in the transmitted signals for minimizing the effects of cochannel NTSC interference in the HDTV receivers. These circuits are well known in the prior art and have the disadvantage of reducing the signal-to-noise ratio of the received signal by about 3 dB. Consequently, unless there is a need to use the filter (i.e., there is significant NTSC signal interference), the filter is not desirable. Some form of decision making circuit is incorporated in the receiver to essentially compare errors with and without the filtering and to disable the filter if there is no substantial benefit.
The receiver also includes a signal equalizer for neutralizing certain types of interference, especially ghost signals. The equalizer includes a series of delays and corresponding multipliers through which the symbols of the received signal are passed. The coefficients of the multipliers are changed in accordance with the determined equalization of the received signal. These circuits and their operation are well known in the art. The prior art circuits make the determination as to whether to include the NTSC filter or not based upon unequalized signals. This has a disadvantage since intersymbol interference may be present in the received signals (due primarily to ghosting) and result in erroneous decisions. The present invention makes the decision based upon equalized data and therefore enables a better decision to be made regarding whether the filter should be included in processing the received signal.